1. Field of the Invention
The present invention relates to gas turbine engine fuel nozzles and, more particularly, to such fuel nozzles having pilot fuel injector tips containing pilot fuel nozzles.
2. Description of Related Art
Aircraft gas turbine engine staged combustion systems have been developed to limit the production of undesirable combustion product components such as oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO) particularly in the vicinity of airports, where they contribute to urban photochemical smog problems. Gas turbine engines also are designed to be fuel efficient and have a low cost of operation. Other factors that influence combustor design are the desires of users of gas turbine engines for efficient, low cost operation, which translates into a need for reduced fuel consumption while at the same time maintaining or even increasing engine output. As a consequence, important design criteria for aircraft gas turbine engine combustion systems include provisions for high combustion temperatures, in order to provide high thermal efficiency under a variety of engine operating conditions, as well as minimizing undesirable combustion conditions that contribute to the emission of particulates, and to the emission of undesirable gases, and to the emission of combustion products that are precursors to the formation of photochemical smog.
One mixer design that has been utilized is known as a twin annular premixing swirler (TAPS), which is disclosed in the following U.S. Pat. Nos. 6,354,072; 6,363,726; 6,367,262; 6,381,964; 6,389,815; 6,418,726; 6,453,660; 6,484,489; and, 6,865,889. It will be understood that the TAPS mixer assembly includes a pilot mixer which is supplied with fuel during the entire engine operating cycle and a main mixer which is supplied with fuel only during increased power conditions of the engine operating cycle. While improvements in the main mixer of the assembly during high power conditions (i.e., take-off and climb) are disclosed in patent applications having Ser. Nos. 11/188,596, 11/188,598, and 11/188,470, modification of the pilot mixer is desired to improve operability across other portions of the engine's operating envelope (i.e., idle, approach and cruise) while maintaining combustion efficiency. To this end and in order to provide increased functionality and flexibility, the pilot mixer in a TAPS type mixer assembly has been developed and is disclosed in U.S. Pat. No. 7,762,073, entitled “Pilot Mixer For Mixer Assembly Of A Gas Turbine Engine Combustor Having A Primary Fuel Injector And A Plurality Of Secondary Fuel Injection Ports” which issued Jul. 27, 2010. This patent is owned by the assignee of the present application and hereby incorporated by reference.
U.S. patent application Ser. No. 12/424,612 (PUBLICATION NUMBER 20100263382), filed Apr. 16, 2009, entitled “DUAL ORIFICE PILOT FUEL INJECTOR” discloses a fuel nozzle having first second pilot fuel nozzles designed to improve sub-idle efficiency, reduced circumferential exhaust gas temperature (EGT) variation while maintaining a low susceptibility to coking of the fuel injectors. This patent application is owned by the assignee of the present application and hereby incorporated by reference.
It is highly desirable to improve the operating efficiency of fuel nozzles. More particularly, it is highly desirable to optimize pilot air flow rate, and strengthen pilot inner swirl number. It is also highly desirable to minimize flow obstruction (leading to flow asymmetry) and maximize pilot air flow-rate within a given fuel nozzle envelope.